One type of apparatus for supplying particulate powder material to dispensing devices such as powder spray guns includes a powder feed hopper having a fluidized bed carrying particulate powder material, and a powder pump mounted exteriorly of the feed hopper. The powder pump is effective to withdraw particulate powder material from the fluidized bed through a siphon tube connected to the inlet of a venturi pumping chamber within the body of the powder pump. A flow of relatively low pressure air is directed into the venturi pumping chamber from an inlet in the pump body which creates a vacuum or suction within the pumping chamber, and, in turn, the siphon tube, to withdraw particulate powder material from the feed hopper. The powder material is entrained in air in the course of movement into the venturi pumping chamber, and this air-entrained powder stream is then directed to a powder dispensing device such as a spray gun for application onto a substrate.
A number of applications require the intermittent supply of particulate powder material to spray guns or other dispensing devices instead of a continuous flow of powder material. In these applications, the flow of pressurized air into the venturi pumping chamber of the powder pump which creates a suction therein must be pulsed or intermittently interrupted so that the powder material is withdrawn from the feed hopper at selected intervals or pulses for supply to the powder spray device. In many powder pump designs, an intermittent supply of pressurized air to the venturi pumping chamber is obtained by operation of the valve located in a relatively long air supply line which is connected between an inlet to the pump body and a source of pressurized air. The valve is intermittently moved between an open position to permit the passage of pressurized air from the valve, through the air supply line to the powder pump, and a closed position to prevent the passage of air therethrough.
Powder pumping apparatus of the type described above have a serious deficiency in applications wherein it is desired to supply powder intermittently to powder dispensing devices. It has been observed that the relatively large open space or "dead zone" contained in that portion of the air supply tube which extends between the valve and the inlet to the pump body results in the production of uneven powder pulses from the powder pump. It is believed that such uneven powder pulses can be attributed to a "tailing" effect created by the air supply line wherein a large amount of air pressure is produced at the beginning of an air pulse, i.e., when the valve is opened to introduce pressurized air through the air supply line into the venturi pumping chamber of the powder pump, and then a gradual tapering off of the air pressure occurs at the end of a pulse when the valve is closed. This surge of high pressure air at the beginning of a pulse and tapering off of the air pressure at the end of a pulse produces a powder pulse or cloud having a "tear drop" shape, wherein a denser powder cloud having a relatively high ratio of powder-to-air is produced at the beginning of the powder pulse and a significantly less dense powder cloud having a comparatively low ratio of powder-to-air is produced at the end of the powder pulse thus forming the "tail portion" of the tear drop shape cloud.
In many powder spraying applications, it is desirable to produce a sharp, well-defined powder pulse having a substantially homogeneous powder-to-air density throughout the duration of the pulse. The "tear drop" shaped powder pulse, with a greater concentration of powder at the beginning of the pulse than at the end, is unacceptable for such applications.